ARROW-17131: [Python] add public-facing field() to UnionType

python/pyarrow/tests/test_types.py

@@ -643,7 +643,7 @@ def test_struct_duplicate_field_names():
 def test_union_type():
     def check_fields(ty, fields):
         assert ty.num_fields == len(fields)
-        assert [ty[i] for i in range(ty.num_fields)] == fields
+        assert [ty.field(i) for i in range(ty.num_fields)] == fields
 
     fields = [pa.field('x', pa.list_(pa.int32())),
               pa.field('y', pa.binary())]

python/pyarrow/types.pxi

@@ -622,9 +622,28 @@ cdef class UnionType(DataType):
         for i in range(len(self)):
             yield self[i]
 
+    def field(self, i):
+        """
+        Return a child field by its numeric index.
+
+        Parameters
+        ----------
+        i : int
+
+        Returns
+        -------
+        pyarrow.Field
+        """
+        if isinstance(i, int):
+            return DataType.field(self, i)
+        else:
+            raise TypeError('Expected integer')
+
     def __getitem__(self, i):
         """
         Return a child field by its index.
+
+        Alias of ``field``.
         """
         return self.field(i)